Assisted stack anode purge at start-up of fuel cell system

ABSTRACT

A fuel cell system that enables an assisted anode purge upon start-up is provided. The fuel cell system includes a fuel cell stack having a plurality of fuel cells with anodes and cathodes. The fuel cell stack has an anode supply manifold and an anode exhaust manifold in fluid communication with the anodes. The fuel cell system further includes a suction device in fluid communication with at least one of the anode supply manifold and the anode exhaust manifold. The suction device adapted to selectively draw a partial vacuum on the fuel cell stack during a start-up of the fuel cell system. Methods for starting the fuel cell system are also provided.

FIELD OF THE INVENTION

The present disclosure relates to a fuel cell system and, moreparticularly, to a start-up system and method for purging anodes of afuel cell stack at start-up.

BACKGROUND OF THE INVENTION

A fuel cell has been proposed as a clean, efficient and environmentallyresponsible power source for electric vehicles and various otherapplications. In particular, the fuel cell has been identified as apotential alternative for the traditional internal-combustion engineused in modern vehicles.

One type of fuel cell is known as a proton exchange membrane (PEM) fuelcell. The PEM fuel cell typically includes three basic components: acathode electrode, an anode electrode, and an electrolyte membrane. Thecathode and anode electrodes typically include a finely dividedcatalyst, such as platinum, supported on carbon particles and mixed withan ionomer. The electrolyte membrane is sandwiched between the cathodeand the anode to form a membrane-electrode-assembly (MEA). The MEA isoften disposed between porous diffusion media (DM) which facilitate adelivery of gaseous reactants, typically hydrogen and oxygen, for anelectrochemical fuel cell reaction.

Individual fuel cells can be stacked together in series to form a fuelcell stack. During start-up of the fuel cell stack, hydrogen gas istypically used to purge the anodes of air that diffuses into andaccumulates on the anodes during shut-down. The flowing of hydrogen gasinto the anodes after a shut-down creates a “hydrogen-air front” thattravels across the anodes. The purge is desirably rapid to minimize theknown carbon degradation that occurs as the hydrogen-air front movesacross the anodes while air is on the cathodes. A conventional fuel cellsystem primarily employs the hydrogen gas pressure during the purge todisplace the accumulated air. However, the rate of fill can be limitedby pressure limitations of the fuel cell stack and flow resistancesacross the fuel cell system.

To mitigate carbon degradation, a short circuit of the fuel cell stackis sometimes performed during the purge. However, carbon corrosion mayalso be caused by a non-simultaneous delivery of hydrogen to the fuelcells. For example, the fuel cells nearest the hydrogen supply mayreceive hydrogen first, and the short circuit is not effective untilmost of the fuel cells have received hydrogen. Thus, the fuel cells thatreceive hydrogen first may experience unmitigated corrosion due to thehydrogen-air front. Additionally, when many of the fuel cells begin toreceive hydrogen, the short circuit begins to operate. However, the fuelcells that do not have hydrogen may experience a negative voltage in aphenomenon known as “cell reversal.” Cell reversal also results in anundesirable carbon corrosion of the fuel cell stack.

Air is also bypassed to an exhaust of a fuel cell stack during start-upto dilute exhausted purge hydrogen. Vehicle emissions standardsgenerally require the exhausted hydrogen concentration to be less thanfour percent (4%) by volume. However, due to the inconsistent conditionsof the fuel cell system following a shut-down period, such as a variablequantity of accumulated air on the anodes, known fuel cell systems arenot particularly effective in optimizing hydrogen emissions duringstart-up.

There is a continuing need for a fuel cell system and method thatprovide an efficient start-up while meeting desired hydrogen exhaustemissions standards. Desirably, the fuel cell system and methods providea rapid system start-up with minimal stack degradation by optimizing thehydrogen-air front time during the start-up.

SUMMARY OF THE INVENTION

In concordance with the instant disclosure, a fuel cell system andmethod that provide an efficient start-up that meets hydrogen exhaustemission standards and minimizes hydrogen-air front time and stackdegradation, is surprisingly discovered.

In one embodiment, a fuel cell system includes a fuel cell stack havinga plurality of fuel cells with anodes and cathodes. The fuel cell stackhas an anode supply manifold and an anode exhaust manifold in fluidcommunication with the anodes. A suction device is in fluidcommunication with at least one of the anode supply manifold and theanode exhaust manifold. The suction device is adapted to selectivelydraw a partial vacuum on the fuel cell stack during a start-up of thefuel cell system.

In another embodiment, a first method for starting the fuel cell systemincludes the step of providing the fuel cell stack having the anodesupply manifold in fluid communication with a first purge valve and ananode inlet valve adapted to selectively deliver hydrogen to the anodesupply manifold. The anode exhaust manifold is in fluid communicationwith an anode outlet valve. A suction device in fluid communication withthe first purge valve and the anode outlet valve is also provided. Thefirst method further includes the steps of: drawing a partial vacuum onthe fuel cell stack by opening at least one of the first purge valve andthe anode outlet valve; closing the anode outlet valve; purging theanode supply manifold with hydrogen by opening the anode inlet valve;closing the first purge valve when the anode supply manifold issubstantially filled with hydrogen; supplying the anodes and the anodeexhaust manifold with hydrogen by opening the anode outlet valve; andsupplying air to the cathodes. The fuel cell stack is thereby placed inan operational mode.

In a further embodiment, a second method for starting the fuel cellsystem includes the step of providing the fuel cell stack having theanode supply manifold in fluid communication with a first purge valve,and an anode inlet valve adapted to selectively deliver hydrogen to theanode supply manifold. The anode exhaust manifold is in fluidcommunication with a second purge valve and an anode outlet valve. Asuction device is also provided in fluid communication with the firstpurge valve and the second purge valve. The second method furtherincludes the steps of: drawing a partial vacuum on the anode supplymanifold by opening the first purge valve; purging the anode supplymanifold with hydrogen by opening the anode inlet valve; closing thefirst purge valve when the anode supply manifold is substantially filledwith hydrogen; drawing a partial vacuum on the anode exhaust manifold byopening the second purge valve and the anode outlet valve, whereinhydrogen is supplied to the anodes and the anode exhaust manifold;closing the second purge valve when the anodes are substantially filledwith hydrogen; and supplying air to the cathodes of the fuel cell stack.The fuel cell system is thereby placed in an operational mode.

DRAWINGS

The above, as well as other advantages of the present disclosure, willbecome readily apparent to those skilled in the art from the followingdetailed description, particularly when considered in the light of thedrawings described hereafter.

FIG. 1 illustrates a schematic, exploded perspective view of a PEM fuelcell stack of the present disclosure, showing only two cells;

FIG. 2 is a schematic, sectional view of the fuel cell stack shown inFIG. 1, showing a plurality of fuel cells in fluid communication with aninlet valve, an outlet valve, and purge valves;

FIG. 3 is a schematic flow diagram of a fuel cell system according to anembodiment of the present disclosure, with alternative connectionsindicated by dashed lines;

FIG. 4 is a schematic flow diagram of the fuel cell system shown in FIG.3, having an eductor adapted to assist an anode purge of the fuel cellsystem, with alternative connections indicated by dashed lines; and

FIG. 5 is a schematic flow diagram of the fuel cell system shown in FIG.3, having an air compressor and flow restrictor adapted to assist ananode purge of the fuel cell system.

DETAILED DESCRIPTION OF THE INVENTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould also be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features. Inrespect of the methods disclosed, the steps presented are exemplary innature, and thus, are not necessary or critical.

FIG. 1 depicts a fuel cell stack 2 having a pair of MEAs 4, 6 separatedfrom each other by an electrically conductive bipolar plate 8. Forsimplicity, only a two-cell stack (i.e. one bipolar plate) isillustrated and described in FIG. 1, it being understood that a typicalfuel cell stack will have many more such cells and bipolar plates.

The MEAs 4, 6 and bipolar plate 8 are stacked together between a pair ofclamping plates 10, 12 and a pair of unipolar end plates 14, 16. Theclamping plates 10, 12 are electrically insulated from the end plates14, 16 by a gasket or a dielectric coating (not shown). The unipolar endplates 14, both working faces of the bipolar plate 8, and the unipolarend plate 16 include flow fields 18, 20, 22, 24. The flow fields 18, 20,22, 24 distribute hydrogen gas and air over an anode and a cathode,respectively, of the MEAs 4, 6.

Nonconductive gaskets 26, 28, 30, 32 provide seals and an electricalinsulation between the several components of the fuel cell stack 2.Gas-permeable diffusion media 34, 36, 38, 40 abut the anodes and thecathodes of the MEAs 4, 6. The end plates 14, 16 are disposed adjacentthe diffusion media 34, 40, respectively, while the bipolar plate 8 isdisposed adjacent the diffusion media 36 on the anode face of MEA 4. Thebipolar plate 8 is further disposed adjacent the diffusion media 38 onthe cathode face of MEA 6.

The bipolar plate 8, unipolar end plates 14, 16, and the gaskets 26, 28,30, 32 each include an anode supply aperture 72 and an anode exhaustaperture 74, a cathode supply aperture 76 and a cathode exhaust aperture78, and a coolant supply aperture 80 and a coolant exhaust aperture 82.Supply manifolds, exhaust manifolds, and coolant manifolds of the fuelcell stack 2 are formed by an alignment of the respective apertures 72,74, 76, 78, 80, 82 in the bipolar plate 8, unipolar end plates 14, 16,and the gaskets 26, 28, 30, 32.

The anode supply apertures 72 and the anode exhaust apertures 74 are influid communication with an anode inlet conduit 84 and an anode outletconduit 86, respectively. A cathode inlet conduit 88 and a cathodeoutlet conduit 90 are in fluid communication with the cathode supplyapertures 76 and the cathode exhaust apertures 78, respectively. Thecoolant supply apertures 80 and the coolant exhaust apertures 82 are influid communication with a coolant inlet conduit 92 and a coolant outletconduit 94, respectively. It should be understood that theconfigurations of the various inlets 84, 88, 92 and outlets 86, 90, 94in FIG. 1 are for the purpose of illustration, and other configurationsmay be chosen as desired.

As shown in FIG. 2, the fuel cell stack 2 may include a plurality offuel cells 200. The assembled fuel cell stack 2 has an anode supplymanifold 202 and an anode exhaust manifold 204. The anode supplymanifold 202 is adapted to receive a flow of hydrogen via the anodeinlet conduit 84 and deliver the hydrogen to the anodes of the pluralityof fuel cells 200. The anode exhaust manifold 204 is adapted to receivethe flow of hydrogen from the anodes of the plurality of fuel cells 200and deliver the hydrogen to the anode outlet conduit 86. The fuel cellstack 2 may further include at least one of a supply manifold purgeconduit 206 in fluid communication with the anode supply manifold 202,and an exhaust manifold purge conduit 208 in fluid communication withthe anode exhaust manifold 204.

In particular embodiments, the anode supply conduit 84 is in fluidcommunication with an anode inlet valve 210. An anode outlet valve 212is in fluid communication with the anode outlet conduit 86. A firstpurge valve 214 is in fluid communication with the supply manifold purgeconduit 206. The first purge valve 214 facilitates a purge of gases fromthe anode supply manifold 202 when in an open position. A second purgevalve 216 may be in fluid communication with the anode exhaust manifold204. The second purge valve 216 facilitates a purge of gases from theanode exhaust manifold when in an open position. It should beappreciated that the anode outlet valve 212, the first purge valve 214,and the second purge valve 216 may be employed, individually or in anycombination, in a purge of the anodes of the fuel cell stack 2 asdesired.

FIG. 3 shows a fuel cell system 300 according to an embodiment of theinvention. The fuel cell system 300 includes the fuel cell stack 2. Thefuel cell stack 2 includes the anode inlet conduit 84 and the anodeoutlet conduit 86, and the cathode inlet conduit 88 and the cathodeoutlet conduit 90. Each of the inlet conduits 84, 88 and outlet conduits86, 90 is in fluid communication with the respective anodes and cathodesof the plurality of fuel cells 200. In a particular embodiment, the fuelcell stack 2 is connected to an electrical load, such as an electricaldrive motor (not shown) of an electric vehicle.

The fuel cell system 300 includes an air compressor 302 in fluidcommunication with the cathode inlet conduit 88. The air compressor 302is adapted to receive air, for example, from the ambient atmosphere andsupply the air to the cathodes of the fuel cell stack 2. The fuel cellsystem 300 also includes a hydrogen source 304 configured to supplyhydrogen gas to the anodes of the fuel cell stack 2. As a nonlimitingexample, the hydrogen source 304 may be a high-pressure storage vesselwith compressed hydrogen gas. It should be understood that othersuitable hydrogen sources 304 may be employed as desired.

The anode inlet valve 210 is disposed between the hydrogen source 304and the anode inlet 84 of the fuel cell stack 2. The anode inlet valve210 is adapted to selectively supply hydrogen from the hydrogen source304 to the anodes of the fuel cell stack 2.

The fuel cell system 300 further includes a bypass valve 306 disposedbetween the air compressor 302 and the fuel cell stack 2. The bypassvalve 306 is adapted to selectively direct a flow of the air from theair compressor 302 around the fuel cell stack 2. In one embodiment, thebypass valve 306 directs the flow of air from the air compressor 302 toan exhaust. The air intermixes with and dilutes residual hydrogen andproducts exhausted from the anodes of the fuel cell stack 2.

An air supply valve 308 may be provided in fluid communication with theair compressor 302 and the fuel cell stack 2. The air supply valve 308may be employed in addition to the bypass valve 306 for purpose ofcontrolling the air flow to the fuel cell stack 2. For example, thebypass valve 306 and the air supply valve 308 may be transitioned tomilitate against excess hydrogen emissions. It should be appreciatedthat while the fuel cell stack 2 is being filled, hydrogen can movethrough the polymer electrolyte membrane to the cathodes, for example,via diffusion and electrochemical pumping. Upon flowing air to the fuelcell stack 2, the hydrogen in the cathodes is forced out to the exhaust.By overlapping the opening and closing of the bypass valve 306 and theair supply valve 308, a quantity of air may be provided to the exhaustof the fuel cell stack 2 sufficient to dilute the hydrogen exiting thecathodes during the filling of the cathodes with air.

In a particular embodiment of the present disclosure, the fuel cellsystem 300 includes a suction device 310. The suction device 310 is influid communication with at least one of the anode supply manifold 202and the anode exhaust manifold 204 of the fuel cell stack 2. The suctiondevice 310 is adapted to selectively draw at least a partial vacuum onthe fuel cell stack 2 during a start-up operation of the fuel cellsystem 300. The suction device 310 creates a vacuum below the ambientpressure which assists the filling of the fuel cell stack 2 withhydrogen. For example, the suction device 310 may provide a vacuum of atleast about 5 kPa lower than ambient pressure. In another nonlimitingexample, the suction device 310 provides a vacuum of up to about 40 kPalower than ambient pressure. It should be appreciated that othersuitable vacuums may be employed as desired.

The suction device 310 may be adapted to selectively draw a partialvacuum on at least one of the anode supply manifold 202 and the anodeexhaust manifold 204, for example. The suction device 310 may also beadapted to selectively draw the partial vacuum on the fuel cell stack 2as a whole, i.e., a simultaneous drawing of a partial vacuum on theanodes and both the anode supply manifold 202 and the anode exhaustmanifold 204. The suction device 310 is particularly adapted to assist apurging of the manifolds and filling of the anodes of the fuel cellstack 2 with hydrogen during the start-up operation.

The suction device 310 is in fluid communication with at least one ofthe anode outlet valve 212, the first purge valve 214, and the secondpurge valve 216. It should be understood that, when one of the anodeoutlet valve 212, the first purge valve 214, and the second purge valve216 are in an open position, the suction device 310 may draw a partialvacuum on the fuel cell stack 2. Likewise, when all of the first purgevalve 214, the second purge valve 216, and the anode outlet valve 212 incommunication with the suction device 310 are in a closed position, thesuction device 310 is unable to draw the partial vacuum on the fuel cellstack 2. Thus, the actuation of the valves 212, 214, 216 is employed toselectively draw the partial vacuum on the fuel cell stack 2.

As a nonlimiting example, when hydrogen is being supplied via the anodeinlet valve 210, the first purge valve 214 is in an open position, andthe other valves 212, 216 (if present) are closed, the partial vacuummay be drawn substantially exclusively on the anode supply manifold 202.As a further nonlimiting example, when the first purge valve 214 and theanode outlet valve 212 are in open positions, the partial vacuum may bedrawn on the fuel cell stack 2 as a whole, including the anode supplymanifold 202, the anode exhaust manifold 204, and the anodes of theplurality of fuel cells 200.

The fuel cell system 300 of the present disclosure may further have atleast one stack shorting device (not shown) in electrical communicationwith the fuel cell stack 2. In particular embodiments, the stackshorting device is a resistor. The stack shorting device is adapted toplace a resistive load on the fuel cell stack 2 during start-up, therebymilitating against fuel cell degradation induced by carbon corrosion. Asuitable stack shorting device is described in assignee's co-pendingU.S. application Ser. No. 11/684,302, incorporated herein by referencein its entirety. Other suitable stack shorting devices may be used asdesired.

An anode recycle pump (not shown) may also be employed in the fuel cellsystem 300. One suitable anode recycle pump is described in assignee'sco-pending U.S. application Ser. No. 11/671,017, incorporated herein byreference in its entirety. The anode recycle pump may be in fluidcommunication with the anode supply manifold 202 and the anode exhaustmanifold 204. The anode recycle pump is adapted to recycle residualhydrogen exhausted from the fuel cell stack 2 in operation. The anoderecycle pump delivers the residual hydrogen back to the anode supplymanifold 202 where it may be used in the fuel cell stack 2electrochemical reactions.

The fuel cell system 300 may also have an anode bleed valve (not shown)configured to discharge accumulated nitrogen in the fuel cell stack 2.The nitrogen may accumulate, for example, due to polymer electrolytemembrane crossover by the cathode air and a recycling of the anodeexhaust with residual hydrogen to the anode supply manifold 202 via theanode recycle pump. In particular embodiments, at least one of the anodeoutlet valve 212, the first purge valve 214, and the second purge valve216 may be employed as the anode bleed valve.

The fuel cell system 300 may employ other fuel cell system componentsknown in the art. For example, the fuel cell system may include at leastone of a humidity sensor, a voltage sensor, a pressure sensor, a watervapor transfer device, a controller, a back-pressure valve, and acharge-air cooler, for example. In a particular embodiment, the fuelcell system 300 includes a plurality of similarly configured fuel cellstacks 2.

Further embodiments of the fuel cell system 300 are shown in FIGS. 4 and5. Like or related structures repeated from FIGS. 1 to 3 include thesame reference numerals with a prime symbol (′) or double-prime symbol(″).

In one embodiment depicted in FIG. 4, the suction device 310′ of thefuel cell system 300′ is an eductor such as a jet pump, venturi nozzle,or aspirator, for example. The suction device 310′ has a motive port, adischarge port, and a suction port. The suction device 310′ isconfigured to receive a motive flow of a fluid, such as an air stream,through the motive and discharge ports and create suction at the suctionport. The suction is capable of drawing a partial vacuum on the fuelcell stack 2 as desired.

The motive port of the suction device 310′ is in fluid communicationwith a motive flow generator, such as the air compressor 302 that isused to supply air to the fuel cell stack 2 while in operation. Themotive flow generator may be a second air compressor, however. It shouldbe appreciated that other suitable motive flow generators may also beemployed.

In one particular example, the motive port of the suction device 310′ isin fluid communication with the bypass valve 306 and the air compressor302. When the bypass valve 306 is configured to bypass a flow of airaround the fuel cell stack 2, such as during the start-up operation, theair flow is directed through the motive port of the suction device 310′.The air flow provides the motive force that enables suction at thesuction port. The air flow is then directed out of the discharge port,along with gases drawn in at the suction port, to the exhaust of thefuel cell system 300′.

A skilled artisan should understand that the suction device 310′ may beadapted to provide a ratio of suction hydrogen flow to motive air flowsuch that the concentration of hydrogen being discharged from thesuction device 310′ is less than a lower flammability limit (LFL) ofhydrogen in air. In a particular embodiment, the suction device 310′provides a ratio of hydrogen flow to motive air flow that results in anexhausted hydrogen concentration of less than about four percent (4%) byvolume.

As shown in FIG. 5, the suction device 310″ of the fuel cell system 300″includes the air compressor 302 and a flow restrictor 500, such as anair filter configured to inhibit a flow of air being drawn therethroughby the air compressor 302. Other suitable flow restrictors 500 may beemployed as desired, such as a flow-restricting valve, for example. Theair compressor 302 is in fluid communication with the flow restrictor500. A reduced pressure zone 502 is formed between the flow restrictor500 and the air compressor 302 during operation of the air compressor302 at start-up. It should be appreciated that, as the flow restrictor500 militates against a flowing of air into the air compressor 302, adepressed pressure may be produced that is sufficient to draw a partialvacuum on the fuel cell stack 2. The fuel cell system 300″ includes thebypass valve 306 adapted to selectively direct the flow of the air fromthe air compressor 302 around the fuel cell stack 2 and to an exhaust,for example.

The present disclosure includes a first method of starting the fuel cellsystem 300. The method includes the step of providing the fuel cellsystem 300. For example, the fuel cell system 300 may have the fuel cellstack 2 with the anode supply manifold 202 and the anode exhaustmanifold 204 in fluid communication with the anodes. The anode supplymanifold 202 of the fuel cell stack 2 is in fluid communication with ananode inlet valve adapted to selectively deliver hydrogen from thehydrogen source 304 to the anode supply manifold. The anode supplymanifold 202 is also in fluid communication with the first purge valve214. The anode exhaust manifold 204 is in fluid communication with theanode outlet valve 212. The provided fuel cell system 300 furtherincludes the suction device 310 in fluid communication with the firstpurge valve 214 and the anode outlet valve 212. The suction device 310is adapted to draw the partial vacuum on the fuel cell stack 2 asdesired.

According to the first method, at least one of the anode outlet valve212 and the first purge valve 214 are opened. In a particularembodiment, both the anode outlet valve 212 and the first purge valve214 are opened concurrently. In opening at least one of the anode outletvalve 212 and the first purge valve 214 in fluid communication with thesuction device, the partial vacuum is drawn on the fuel cell stack 2 asa whole. It should be appreciated that opening both the anode outletvalve 212 and the first purge valve 214 militates against a pulling ofgases from one of the anode supply manifold 202 and the anode exhaustmanifold 204 into the anodes of the fuel cells 200. Thus, a traveling ofhydrogen-air fronts across the anodes is militated against by initiallydrawing the partial vacuum on both the anode supply manifold 202 and theanode exhaust manifold 204 sides of the fuel cell stack 2.

The method next includes a step of closing the anode outlet valve 212.The anode inlet valve 210 is then opened, resulting in a purging of theanode supply manifold 202. During the purge step, it should beappreciated that hydrogen may be supplied through the anode inlet valve210 at a flow rate that militates against a flow of hydrogen into theanodes of the fuel cells 200. For example, the hydrogen is supplied at arate sufficient to maintain the internal vacuum on the fuel cell stack2. The internal vacuum on the fuel cell stack may be substantially equalto the vacuum established prior to introduction of hydrogen via theanode inlet valve 210. The hydrogen is drawn through the anode supplymanifold 202 and the first purge valve 214 in communication with thesuction device 310, thereby militating against any substantial migrationof the hydrogen into the anodes of the fuel cell stack 2. The durationof the purge step is a time adequate to substantially fill the anodesupply manifold 202 with the hydrogen gas.

Following the purging of the anode supply manifold 202, the first purgevalve 214 is closed and the anode outlet valve 212 is opened. The anodesof the fuel cells 200 and the anode exhaust manifold 204 are then purgedwith hydrogen in a “stack fill” step. The flow rate of the hydrogen maybe increased upon opening the anode outlet valve 212 to facilitate arapid fill of the anodes and the anode exhaust manifold 204. It shouldbe understood that the filling of the anodes and the anode exhaustmanifold 204 is assisted during this step by the drawing of the partialvacuum at the anode outlet valve 212. The duration of the stack fillstep is a time sufficient to substantially fill the anodes of the fuelcells 200 with hydrogen.

To complete the start-up of the fuel cell system 300, air is supplied tothe cathodes of the fuel cells 200. For example, the air is supplied bythe air compressor 302. In the step of supplying air to the cathodes, atleast one of the bypass valve 306 and air supply valve 308 may beadapted to divert air to the cathode inlet conduit 88 of the fuel cellstack 2.

With renewed reference to FIG. 4, the first method may employ thesuction device 310′, such as the eductor having the motive port, thedischarge port, and the suction port. The suction port may be in fluidcommunication with at least one of the anode outlet valve 212 and thefirst purge valve 214. The air compressor 302 may also be provided influid communication with the motive port of the suction device 310′. Inoperation, the method therefore includes the step of starting the aircompressor 302 to provide the motive air flow to the suction device310′. The air compressor 302 is started prior to the drawing of thepartial vacuum on the fuel cell stack 2, so that the suction device 310′may generate suction sufficient to draw the partial vacuum.

The method may further include the steps of providing the stack shortingdevice in electrical communication with the fuel cell stack 2. The stackshorting device is engaged before the filling of the anodes withhydrogen. An electrical load is thereby applied to the fuel cell stack 2that militates against carbon corrosion of the fuel cell stack 2 whilethe anodes are filled with hydrogen. After the anodes are substantiallyfilled with hydrogen, the stack shorting device may be disengaged.

A skilled artisan should understand that a partial vacuum is typicallycreated within the anodes of the fuel cell stack 2 during a conventionalshut-down. As hydrogen is consumed without replenishment, the anodegases cool and water vapor condenses, resulting in a sub-ambientpressure on the anodes, the anode supply manifold 202, and the anodeexhaust manifold 204. Thus, in conventional systems, the opening of theanode outlet valve 212 allows a back-flow of air from the atmosphereinto the anodes having the sub-ambient pressure. The backflow creates anundesirable hydrogen-air front that may result in carbon corrosion and adegradation of performance. The fuel cell system 300, 300′ and the firstmethod of the disclosure militates against a back-flow of air into thefuel cell stack 2 by filling the anodes with hydrogen.

The present disclosure also includes a second method for starting thefuel cell system 300. The second method also includes the step ofproviding the fuel cell system 300. For example, the fuel cell stack 2includes the anode supply manifold 202 in fluid communication with theanode inlet valve 210 and the first purge valve 214. In contrast withthe first method, however, the anode exhaust manifold 204 is in fluidcommunication with the anode outlet valve 212 and the second purge valve216. The suction device 310 is also provided in fluid communication withthe first purge valve 214 and the second purge valve 216 and adapted todraw a vacuum on the fuel cell stack 2 as desired.

According to the second method, the suction device 310 draws the vacuumsubstantially exclusively on the anode supply manifold 202 when thefirst purge valve 214 is opened. In operation, a purge step follows thedrawing of the partial vacuum on the anode supply manifold 202. In thepurge step, the anode inlet valve 210 is opened and the anode supplymanifold 202 is substantially filled with hydrogen. After the anodesupply manifold 202 is substantially filled with hydrogen, the firstpurge valve 214 is closed.

The method next includes the step of drawing the partial vacuum on theanode exhaust manifold 204 by opening the anode outlet valve 212 and thesecond purge valve 216. The opening of the valves 212, 216 may beperformed substantially simultaneous with the closing of the first purgevalve 214. As the anode inlet valve 210 is already opened, the openingof valves 212, 216 allows the hydrogen to purge the anodes and the anodeexhaust manifold 204 in the stack fill step. The opening of both theanode outlet valve 212 and the second purge valve 216, both in fluidcommunication with the suction device 310, facilitates a rapid fill ofthe anodes and the anode exhaust manifold 204 with hydrogen. Inparticular, opening up both valves 212, 216 minimizes a resistance toflow of hydrogen through the anodes and the anode exhaust manifold 204.The second purge valve 216 is closed when the anodes are substantiallyfilled with hydrogen, for example.

As with the first method described herein, the start-up of the fuel cellsystem 300 according to the second method is completed when air issupplied to the cathodes of the fuel cell stack 2. With both hydrogenflowing to the anodes and air flowing to the cathodes, the fuel cellsystem 300 is placed in an operational mode.

With renewed reference to FIG. 5, the second method may employ the fuelcell system 300″ having the suction device 310″, e.g., the aircompressor 302 in fluid communication with the flow restrictor 500 andhave a reduced pressure zone 502 disposed therebetween. In thisconfiguration, the first purge valve 214 and the second purge valve 216are in fluid communication with the reduced pressure zone 502. When thesecond method employs the suction device 310″, the method includes thestep of starting the air compressor prior to the step of drawing thepartial vacuum on the anode supply manifold 202. The vacuum is therebygenerated in the reduced pressure zone 502 that is sufficient to drawthe partial vacuum on the fuel cell stack 2.

It should be understood that the fuel cell system 300, 300′, 300″ andmethods described herein employ the selective drawing of the partialvacuum to pull air from the anodes of the fuel cell stack 2 that hasaccumulated during shut-down of the fuel cell system. Thus, the purgingand filling of the anodes with hydrogen is assisted by the drawing ofthe partial vacuum on the fuel cell stack 2. A pressure of hydrogensufficient to displace the accumulated air is thereby reduced,particularly because the partial vacuum assists by pulling theaccumulated air from the fuel cell stack 2.

One of ordinary skill should understand that the fuel cell system 300,300′, 300″ and methods of the disclosure militate against a flow ofhydrogen into the anodes of the plurality of fuel cells 200 during thepurging of the anode supply manifold 202. For example, the partialvacuum drawn on the anode supply manifold 202 before the hydrogen purgestep allows for a rapid fill without exceeding a pressure where thehydrogen is forced into the anodes of the fuel cell stack 2.

The assisted purging and filling of the fuel cell stack 2 with hydrogenvia the drawing of the partial vacuum on at least one of the anodesupply manifold 202 and the anode exhaust manifold 204 also facilitate arapid and reliable start-up. The fuel cell system 300, 300′, 300″ andmethods result in a substantially even distribution of hydrogenthroughout the fuel cell stack 2 anodes. As the hydrogen-air front movesrapidly as a “fast front” in response to both hydrogen pressure and thepartial vacuum drawn on the fuel cell stack 2, cell reversals, negativecell voltages, and carbon corrosion are militated against. A durabilityof the fuel cell stack 2 is thereby optimized.

The fuel cell system 300, 300′, 300″ and methods of the disclosure alsominimize hydrogen emissions. For example, due to the assisting of theanode purge and fill of the fuel cell stack 2 with the partial vacuumbeing drawn thereon, the pressure of hydrogen supplied to the fuel cellstack 2 may be reduced. Conventional fuel cell systems rely on thepressure of hydrogen to displace a combination of accumulated air andresidual hydrogen of unknown composition. The partial vacuum drawn onthe fuel cell stack 2 enables the employment of less hydrogen todisplace the gases present after shut-down. Supplying a sufficientquantity of hydrogen for fuel cell stack 2 operations, while notexceeding an amount that results in exhaust emission greater than aboutfour percent by volume, is thereby facilitated with the present fuelcell system 300, 300′, 300″ and methods.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes may be made without departingfrom the scope of the disclosure, which is further described in thefollowing appended claims.

1. A fuel cell system comprising: a fuel cell stack including aplurality of fuel cells having anodes and cathodes, the fuel cell stackhaving an anode supply manifold and an anode exhaust manifold in fluidcommunication with the anodes; and a suction device in fluidcommunication with at least one of the anode supply manifold and theanode exhaust manifold, the suction device adapted to selectively draw apartial vacuum on the fuel cell stack during a start-up of the fuel cellsystem.
 2. The fuel cell system of claim 1, further comprising at leastone of a first purge valve in fluid communication with the anode supplymanifold, a second purge valve in fluid communication with the anodeexhaust manifold, and an anode outlet valve in fluid communication withthe anode exhaust manifold.
 3. The fuel cell system of claim 2, whereinthe suction device is in fluid communication with the at least one ofthe first purge valve, the second purge valve, and the anode outletvalve.
 4. The fuel cell system of claim 1, wherein the suction device isan eductor having a motive port, a discharge port, and a suction port.5. The fuel cell system of claim 4, wherein at least one of the anodesupply manifold and the anode exhaust manifold is in fluid communicationwith the suction port of the eductor.
 6. The fuel cell system of claim4, wherein the motive port of the eductor is in fluid communication witha motive flow generator.
 7. The fuel cell system of claim 6, wherein themotive flow generator is an air compressor.
 8. The fuel cell system ofclaim 1, wherein the suction device includes an air compressor in fluidcommunication with a flow restrictor.
 9. The fuel cell system of claim8, wherein a reduced pressure zone is formed between the air compressorand the flow restrictor when the air compressor is in operation.
 10. Thefuel cell system of claim 8, wherein the flow restrictor is a filter.11. A method for starting a fuel cell system, the method comprising thesteps of: providing a fuel cell stack with a plurality of fuel cellshaving anodes and cathodes, the fuel cell stack having an anode supplymanifold and an anode exhaust manifold in fluid communication with theanodes, the anode supply manifold in fluid communication with a firstpurge valve and a anode inlet valve adapted to selectively deliverhydrogen to the anode supply manifold, the anode exhaust manifold influid communication with an anode outlet valve; providing a suctiondevice in fluid communication with the first purge valve and the anodeoutlet valve; drawing a partial vacuum on the fuel cell stack by openingat least one of the first purge valve and the anode outlet valve;closing the anode outlet valve; purging the anode supply manifold withhydrogen by opening the anode inlet valve; closing the first purge valvewhen the anode supply manifold is substantially filled with hydrogen;supplying the anodes and the anode exhaust manifold with hydrogen byopening the anode outlet valve; and supplying air to the cathodes,wherein the fuel cell stack is placed in an operational mode.
 12. Themethod of claim 11, wherein the suction device is an eductor having amotive port, a discharge port, and a suction port, the suction port influid communication with at least one of the first purge valve and theanode outlet valve.
 13. The method of claim 12, further comprising thesteps of: providing an air compressor in fluid communication with themotive port of the eductor; and starting the air compressor prior to thestep of drawing a partial vacuum on the fuel cell stack, wherein amotive air flow is provided to the eductor.
 14. The method of claim 13,wherein the anode inlet valve supplies hydrogen at a flow rate thatmilitates against a flow of hydrogen into the anodes during the step ofpurging the anode supply manifold.
 15. The method of claim 13, wherein aratio of a hydrogen flow to the motive air flow results in an exhaustedhydrogen concentration of less than about 4 percent by volume.
 16. Themethod of claim 13, further comprising the steps of: providing a stackshorting device in electrical communication with the fuel cell stack;engaging the stack shorting device before the filling of the anodes withhydrogen, wherein an electrical load is applied on the fuel cell stack;and disengaging the stack shorting device after the anodes aresubstantially filled with hydrogen.
 17. The method of claim 11, furthercomprising the steps of: providing an anode recycle pump in fluidcommunication with the anode supply manifold and the anode exhaustmanifold, the anode recycle pump adapted to recycle residual hydrogenexhausted from the fuel cell stack; causing hydrogen to flow from theanode exhaust manifold around the anode recycle pump before the fillingof the anodes with hydrogen; filling the anode recycle pump withhydrogen; and supplying hydrogen from the anode exhaust manifold to therecycle pump after the anodes and the anode exhaust manifold aresubstantially filled with hydrogen.
 18. A method for starting a fuelcell system, the method comprising the steps of: providing a fuel cellstack with a plurality of fuel cells having anodes and cathodes, thefuel cell stack having an anode supply manifold and an anode exhaustmanifold in fluid communication with the anodes, the anode supplymanifold in fluid communication with a first purge valve and a anodeinlet valve adapted to selectively deliver hydrogen to the anode supplymanifold, the anode exhaust manifold in fluid communication with asecond purge valve and an anode outlet valve; providing a suction devicein fluid communication with the first purge valve and the second purgevalve; drawing a partial vacuum on the anode supply manifold by openingthe first purge valve; purging the anode supply manifold with hydrogenby opening the anode inlet valve; closing the first purge valve when theanode supply manifold is substantially filled with hydrogen; drawing apartial vacuum on the anode exhaust manifold by opening the second purgevalve and the anode outlet valve, wherein hydrogen is supplied to theanodes and the anode exhaust manifold; closing the second purge valvewhen the anodes are substantially filled with hydrogen; and supplyingair to the cathodes, wherein the fuel cell system is placed in anoperational mode.
 19. The method of claim 18, wherein the suction deviceis an air compressor in fluid communication with a flow restrictor andhaving a reduced pressure zone disposed therebetween, the reducedpressure zone in fluid communication with the first purge valve and thesecond purge valve.
 20. The method of claim 19, further comprising thestep of starting the air compressor prior to the step of drawing apartial vacuum on the anode supply manifold, wherein suction isgenerated in the reduced pressure zone.